Pumps used for pumping molten metal typically include a motor carried by a motor mount, a shaft connected to the motor at one end, and an impeller connected to the other end of the shaft. Such pumps may also include a base with an impeller chamber, the impeller being rotatable in the impeller chamber. Support members extend between the motor mount and the base and include a shaft sleeve surrounding the shaft, support posts, and a tubular riser. A volute member may be employed in the impeller chamber. Pumps are designed with pump shaft bearings, impeller bearings and with bearings in the base that surround these bearings to avoid damage of the shaft and impeller due to contact with the shaft sleeve or base. The shaft, impeller, and support members for such pumps are immersed in molten metals such as aluminum, copper, iron and alloys thereof. The pump components that contact the molten metal are composed of a refractory material such as graphite.
The pumps commonly used to pump molten metal are one of three types, transfer pumps, discharge pumps, and circulation pumps, as disclosed in the publication "H.T.S. Pump Equation for the Eighties" by High Temperature Systems, Inc., which is incorporated herein by reference in its entirety.
A transfer pump transfers molten metal out of one furnace to another furnace or into a ladle. The transfer pump has all of the pump components described above. A tubular riser extends vertica from the base chamber to the motor mount and contains a passageway for molten metal. Support posts are also provided between the base and the motor mount.
A discharge pump transfers molten metal from one bath chamber through a submerged pipe to another bath chamber. Such a pump typically includes a shaft sleeve and support posts between the base and the motor mount, but has no riser.
A circulation pump circulates molten metal to improve alloy homogenization, among other things. A circulation pump may also be configured without a base, shaft sleeve, support posts or riser, having only an impeller connected to a shaft sleeve immersed in the molten metal, as described in U.S. Pat. No. 5,143,357 to Gilbert et al., issued Sep. 1, 1992. However, a circulation pump could include all of the pump components described above except a riser.
The operation of all three types of pumps is similar. The pump is lowered into a bath of molten metal and secured in place. The motor is activated and rotates the shaft via a coupling assembly between the shaft and motor. The shaft then rotates the impeller, thereby effecting fluid flow of the molten metal.
In the operation of circulation pumps without a base, the molten metal is then circulated in the furnace by rotation of the impeller on the end of the shaft. In circulation pumps having a base, rotation of the impeller draws molten metal into the impeller chamber of the base and out of an outlet of the base in a desired direction.
In the operation of transfer pumps, rotation of the impeller draws molten metal into the impeller chamber of the base and out a base outlet to the riser. The molten metal then passes through the riser and is removed from the pump.
In the operation of discharge pumps, rotation of the impeller draws molten metal into the inlet and out of the outlet of the base. The molten metal then passes through the submerged pipe into another bath chamber.
Pumps which employ a base may either be top feed pumps or bottom feed pumps depending, among other things, on the configuration of the base and orientation of the impeller vanes relative to the direction of shaft rotation. Multiple impellers and volutes may be used, as disclosed in U.S. Pat. No. 4,786,230 to Thut, which is incorporated herein by reference in its entirety.
Impellers have been used in pumps of the types described with two, three, and as many as four blades. An impeller is connected to one end of a long shaft. The impeller is typically constructed of refractory material by machining. When the impeller is connected to the end of the shaft it invariably vibrates or wobbles. The refractory impeller cannot be produced to eliminate all wobble on the end of the shaft.
Three bladed cylindrical hub impellers have high displacement of molten metal. However, using a three-bladed impeller is disadvantageous in that it is not well balanced and wobbles excessively. The disadvantage of using an impeller with more than three blades is that as more blades are used, displacement of the molten metal is reduced.
In many instances two and four bladed cylindrical hub impellers offer the advantage of better balance and less wobble than three bladed, cylindrical hub impellers. However, two and four bladed cylindrical hub impellers suffer from a repeated impact problem. When pumping molten metal the impeller bearing and shaft bearing repeatedly strike their surrounding bearings at the same radial locations on the periphery of the impeller bearing and shaft bearing. This so-called chatter results in relatively rapid destruction of at least one of the shaft, impeller bearing, and impeller. Typically, the pump shaft is sheared. The short useful lives of the pump shaft, impeller, and impeller bearing lead to increased production costs attributable to frequent replacement of the pump components and consequential down time of the pump.